Engine order cancellation (EOC) technology uses a non-acoustic signal representative of the engine (motor) noise as a reference to synthesize a sound wave that is opposite in phase to the engine noise audible in the car interior. As a result, EOC makes it easier to reduce the use of conventional damping materials. Common EOC systems utilize a narrowband feed-forward active noise control (ANC) framework in order to generate anti-noise by adaptive filtering of a reference signal that represents the engine harmonics to be cancelled. After being transmitted via a secondary path from an anti-noise source to a listening position, the anti-noise has the same amplitude but opposite phase as the signals generated by the engine filtered by a primary path that extends from the engine to the listening position. Thus, at the place where an error microphone resides in the room, for example, at or close to the listening position, the overlaid acoustical result would ideally become zero so that error signals picked up by the error microphone would only record sounds other than the cancelled harmonic noise signals generated by the engine.
Commonly a non-acoustical sensor, for example, a sensor measuring the repetitions-per-minute (RPM), is used as a reference. The signal from the RPM sensor can be used as a synchronization signal for synthesizing an arbitrary number of harmonics corresponding to the engine harmonics. The synthesized harmonics form a basis for noise canceling signals generated by a subsequent narrowband feed-forward ANC system. Even if the engine harmonics mark the main contributions to the total engine noise, they by no means cover all noise components radiated by the engine, such as bearing play, chain slack, or valve bounce. However, an RPM sensor is not able to cover signals other than the harmonics.